Static trip units such as that described within U.S. Pat. No. 4,266,259 entitled "Long-time and Short-time Overcurrent Signal Processor For Circuit Breaker Static Trip Units" to E. K. Howell, employ a current transformer for monitoring the current within a protected circuit. When such a trip unit also includes ground fault protection function and the line sensing transformer is used to sense ground fault currents, a highly accurate current transformer is required. The transformer generally consists of a so-called "wound core" wherein the secondary winding comprises a toroidal winding which is arranged to optimize the transformer efficiency to less than one percent maximum error under calibrated test conditions. A current transformer having a toroidal secondary winding is described, for example, in U.S. Pat. No. 3,449,703 entitled "Current Transformer Having An Accuracy Unimpaired By Stray Flux From Adjacent Conductors" to F. L. Steen.
With the advent of minimum function circuit interrupters without ground fault function wherein the thermal and magnetic response characteristics of molded case circuit breakers are provided within a static trip unit with electronics, such an accurate current sensor is no longer required. The minimum function circuit interrupter employs an analog signal processor similar to that described within the Howell patent to perform long-time, short-time and instantaneous trip functions. It is contemplated that the minimum function static trip unit will be employed within circuit breaker frame ratings lower than those currently utilizing the Howell static trip unit. Further, since ground fault protection is not required, a less expensive current sensing transformer would be highly desirable. It is known that laminated core transformers are less expensive to fabricate than the earlier described wound core and that transformer windings obtained from a simple bobbin machine are substantially less expensive than the earlier described toroidal winding. To date, laminated core current transformers have not proven feasible as current sensors within electronic trip units because of their low accuracy as determined by calibrated test techniques in comparison to the more sophisticated current transformers. The poor performance obtained with laminated core current transformers having simple bobbin secondary windings is caused by the leakage flux through that portion of the core which is exposed between the primary strap and the secondary winding.
The purpose of this invention is to provide a low cost current sensor having good transfer characteristics for minimum function static trip units by utilizing a bobbin wound secondary winding on a laminated transformer core and increasing the flux transfer between the primary strap and the secondary winding.